Spark plug electrode



Patented Jan. 15, 1946 SPARK PLUG ELECTRODE of Delaware No Drawing.

Application December 17, 1943, Se-

rial No. 514,659. In Great Britain November 9,

10 Claims.

This invention relates to spark plug electrodes.

It is common practice in engines of high efilciency to employ spark plug electrodes made from precious metal alloys, as these have particularly good resistance to high temperatures, electrical erosion and the corrosive effects of the products of combustion. It has been found however that when the fuels employed have 'a high lead content, so that a high concentration of lead occurs in the cylinder gas, many of the alloys known to be satisfactory under mild operating conditions are unsuitable since lead is very active at high temperatures in attacking the electrodes, causing corrosion and breakage. It is believed that this attack is due to penetration of the lead along the grain boundaries to the core of the electrode, causing severe deterioration of the alloy, so that the electrode is then liable to break after only a comparatively short time in service.

The principal object of this invention is to provide spark plug electrodes having greater resistance to attack by lead than the electrodes now commonly used.

Another object of this invention is to provide .an improved spark plug electrode containing palladium.

A further object of this invention is to provide a spark plug electrode having a long life and considerable resistance to erosion.

I have discovered that if an alloy containing palladium is used, the lead is adsorbed in the surface layers and diiiuses into the crystals of the alloy surface, so that the lead does not penetrate along the grain boundaries to the core of the electrode wire and the electrode as a whole possesses resistance to lead attack and consequent improved resistance to embrittlement.

The alloys that are used according to the invention consist principally of platinum, since this imparts the best resistance to erosion in combination with the other properties desired. Their palladium content depends largely upon the conditions under which the electrode is to be used. because I have found that the palladium, although increasing the solubility of lead in the alloy and thus increasing the .tlme before the electrode fails in consequence of embrittlement, reduces the capacity of the alloy for resisting erosion, so that if it is essential that the alloys should possess high resistance to erosion and the risk of embrittlement is not great it is better to keep the palladium content low. If, however, the risk. of embrittlement is great, e. g. if the electrodes are to be used with fuels of high lead content and if they are likely to operate at high temperatures, their palladium content should be increased. The electrodes may then erode more quickly, but their effective life is increased as they will not break in consequence of leadat tack. Thus if the risk of embrittlement is substantial the palladium content of a binary alloy may be about 15% or more, amounting even to about 40% in some cases. If, on the other hand, the risk of erosion is substantial then the palladium content should be kept low and preferably between about 5% and about 10%.

Although binary alloys may be used, ternary alloys are more satisfactory when not only resistance to erosion and embrittlement but also mechanical stiffness and strength are desired.

The alloys which I prefer are ternary alloys in which some of the platinum is replaced by tungsten or ruthenium. These elements act as hardeners and impart good mechanical properties to the alloys, but it seems that it is their presence that renders the alloys particularly liable to at tack by lead along the grain boundaries or that they reduce the solubility of lead in the alloy, so I normally prefer to keep them at the minimum necessary for strengthening purposes. For instance, the alloys may contain from about 0.5% to about 5% of tungsten, or from about 0.5% to about 6% of ruthenium at the expense of the platinum content.

Examples of alloys that may be used when the risk of embrittlement is not great are given in Table I.

When in consequence of considerable risk of embrittlement relatively high palladium contents are used, there is an advantage in increasing the tungsten or ruthenium content. The reason is that the recrystallisation temperature of the alloy is then raised, and in consequence a fibrous microstructure can be maintained in the electrode. The advantage of this is fully explained in my application Serial No. 485,704 filed May 5, 1943. For instance, in conjunction with about 10% palladium or more I may use from about 3% to about 8% of tungsten or from about 4% to about 10% otx'uthenium and draw the resultant alloy down sumciently to ensure that the electrodes have a fibrous microstructure.

Examples of alloys that may be used when there is substantial risk of embrittlement. are

iven in Table II. v

Table 11 Alloy Pt Pd W Ru Per 1 21 Per Per cent cent cent cent 4 85 5 5 84 i0 6 ii 74 2o 6 The alloys may be quaternary or even more complex, although the total amount of any elements other than" those already mentioned should not exceed about 10%. Examples of such other elements that may be present are iridium and osmium, which act as hardeners in the same way as tungsten and ruthenium but are not so desirable as these latter elements. Again rhodium is a mild hardening agent and may be used in alloys in which mechanical strength is desired. However as palladium itself is only a mild hardening agent,

' rhodium'is normally only included in alloys con- I taining more than 10% palladium because with lesser palladium contents the combined eifect of palladium and rhodium is insuflicientto bring" such minor quantities of impurities;

I claim:

2,39 ,ecc

2. A spark plug electrode made of a binary alloy containing about 4% to about palladium and the balance substantially all platinum.

3. A spark plug electrode made of an alloy containing more than 10% and up to about 20% palladium, from about 0.5% to about 10% or one metal selected from the group consisting of tungsten and ruthenium, and from 0 to about 10% of other elements, the balance being platinum.

4. A spark plug electrode made of a ternary alloy containing from about 4% to about 10% palladium and from about 0.5% to about 5% tungsten, the balance being platinum.

"l. A spark plug having an electrode made of a rhodium-free alloy containing from about 4% to about 20% palladium, from 0 to about 10% of other elements, the balance being platinum.

5. A spark plug,v electrode made of a ternary alloy containing from about 4% to about 10% palladium and from about 0.5% to 6% ruthenium, the balance being platinum.

6. A spark plug electrode made of a ternary alloy containing from about 10% to about 20% palladium and from about 3% to about 8% tungsaid alloy sten, the balancebeing platinum and having a fibrous microstructure.

7. A spark plug electrode made of a ternary alloy containing from about 10% to about 20% palladium. and from about 4% to about 10% ruthenium, the balance being platinum and said alloy having a fibrous microstructure.

8. A spark plug electrode made of an alloy. containing about 4% to about 20% palladium.

from about 0.5% to about 10% rutheniuman the. balance substantially platinum.

9. A spark plug electrode'made'of an alloy containing about 4% to about 20% palladium.

vfrom about 0.5% to about 8% of tungsten'and the balance substantially platinum.

-10. A spark plug having an electrode made of a rhodium-free alloy containing about 4% to' about 20% palladium, about 0.5% to 8% of one.

metal selected from the group consisting of tungsten and ruthenium, and from 0 to about 10% 

